Our research efforts are directed toward investigating the life cycle and basic functions of various cell types lining pulmonary airways, in a well-defined in vitro condition. Such a culture offers a model system to investigate the mechanism of cell differentiation, as well as the repair of injury and the pathological response of cells to environmental toxic substances. Primary cultures from rabbit, rat, hamster, and mouse tracheal epithelium have been established. Experiments are carried out to determine: 1) the life span of normal epithelial cells in culture, 2) the origin of cultured cells in tracheal epithelium, 3) the fates of various cell types in culture, and 4) differentiated properties. We have observed that rat tracheal epithelial cells lose their differentiated properties immediately after plating, but regain their differentiated function after plating in denuded tracheal graft. Furthermore, rabbit tracheal epithelial cells maintain some properties of mucus production and response to rentinoid and, therefore, the in vitro tracheal epithelial cultures can be used as a model system to study mucus cell differentiation.